A fiber-reinforced composite material, which is used as a light, strong material, is a composite of a fiber structure and a plastic matrix, for example. Thus, a fiber-reinforced composite material has physical properties (mechanical properties) superior to the matrix. A fiber-reinforced composite material may be used as an impact absorber, which is compressed and broken in the direction of the impact load to absorb the impact energy.
For example, a fiber-reinforced composite material having a sectoral shape including arcuate sections in a plan view may be used as an impact absorber. Such a fiber-reinforced composite material may include a fiber structure, an example of which is disclosed in Patent Document 1. Patent Document 1 discloses a fiber element having a sectoral shape in a plan view. The fiber element is fixed to the surface of the fiber structure when forming the preform.
As shown in FIG. 6, a fiber element 80 includes a first arcuate section 81 at one end in the longitudinal direction and a second arcuate section 82 at the other end. The length of the arc of the first arcuate section 81 is less than that of the second arcuate section 82. The fiber element 80 is shaped such that the width increases continuously from the first arcuate section 81 to the second arcuate section 82.
In Patent Document 1, the fiber element 80 is passed through a variable throat 83 shown in FIG. 7 so that the fiber element 80 obtains a width that varies continuously. The variable throat 83 includes a cylindrical bar 84 and circular plates 85 fixed to the opposite ends of the bar 84. Each circular plate 85 has a thickness that varies continuously conforming to the circumference of the bar 84. Thus, the width of the opening of the passing section 86 between the circular plates 85 also varies continuously conforming to the circumference of the bar 84. The fiber element 80 having a uniform width is passed through the passing section 86 of the variable throat 83, resulting in the fiber element 80 having a width that varies continuously.
However, when the width of the fiber element 80 is changed by passing the fiber element 80 through the passing section 86 as described above, the thickness of a section of the fiber element 80 having a smaller width is greater than the thickness of a section having a greater width. The alignment of fibers varies as the width of the fiber element 80 varies. Thus, the physical properties of the fiber structure tend to vary from the first arcuate section 81 toward the second arcuate section 82.